Almost 9m old hasn't pooped in a week...
It might be that his body is just absorbing everything he's taking in. But if he does show signs of discomfort and/or pain you could also try massaging his tummy. Remember to always go in a clockwise direction as this is the way the intestines work. It might also be a good way to feel if his tummy is hard or not?
Banana never constipated any of my three kids, but my sister's daughter gets very constipated from it, so I think that completely depends on each child.
JennaRose I love your P's tip. I've known about the ABC diet for diarhea (apples, bananas, carrots) but the P's one makes sense for constipation. Thanks for sharing!